Low-viscosity polyurethane acrylate dispersions

Abstract

The present invention relates to a process for the preparation of low-viscosity, radiation-curable, aqueous dispersions based on polyurethane acrylates (UV-PUDs), wherein the UV-PUDs hydrophilized via hydroxy-functionalized carboxylic acids have a lower initial viscosity when a large part of the urethanization, that is to say the reaction of the NCO-functionalized compounds with the OH-functionalized compounds, initially takes place in the absence of the acid-carrying compounds and the incorporation of the acid-carrying compounds via their hydroxyl function takes place as late as possible.

The invention claimed is: 
     
       1. A process for preparing a radiation-curable, aqueous dispersion comprising a polyurethane acrylate (i) comprising as structural components
 A) one or more polymeric compounds selected from the group consisting of polyester (meth)acrylates, polyether (meth)acrylates, polyether ester (meth)acrylates, unsaturated polyesters having allyl ether structural units, and polyepoxy (meth)acrylates, wherein the one or more polymeric compound has an OH number in the range from 15 to 300 mg KOH/g of substance; 
 B) optionally one or more monomeric compounds other than A) comprising a hydroxy function and at least one (meth)acrylate group; 
 C) one or more monomeric monools, diols and/or triols, other than A) or B), in each case having a molecular weight of from 32 to 240 g/mol or polyesters, polycarbonates, polyether carbonate polyols, C2-, C3- and/or C4-polyethers, polyether esters, polycarbonate polyesters having a functionality of from 1.0 to 3.0, in each case having a weight-average molar mass Mw in the range from 300 to 4000 g/mol and being other than A) or B); 
 D) one or more organic polyisocyanates selected from the group consisting of 1,6-hexamethylene diisocyanate, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate or IPDI) and 4,4′-diisocyanatodicyclohexylmethane; 
 E) one or more compounds selected from the group consisting of dimethylolpropionic acid and dimethylolbutyric acid; 
 F) one or more compounds selected from the group consisting of mono-amines, di-amines, mono-functional amino alcohols and difunctional amino alcohols; 
 wherein components A) through D) are reacted in a first reaction step to form a polyurethane acrylate which comprises neither ionic nor potentially ionic groups and, after determination of the NCO content, an NCO value is reached which is within 1.5 wt. % NCO (absolute) of the theoretical NCO content, wherein the molar ratio of isocyanate groups in D) to groups reactive towards isocyanates in A) to C) is from 1.2:1.0 to 4.0:1.0; and 
 in a second reaction step, component E) is reacted with free NCO groups from the reaction product of components A) to D) and, after again determining the NCO content, an NCO value is reached which is within 1.5 wt. % NCO (absolute) of the theoretical NCO content, wherein the molar ratio of isocyanate groups in D) to groups reactive towards isocyanates in A), B), C) and E) is from 1.2:1.0 to 1.5:1.0, wherein a neutralising agent is added during or after the second reaction step; and 
 in a third reaction step, component F) is reacted with free NCO groups, 
 wherein a dispersing step is carried out after the second step or, if the neutralisation is carried out after the second step, after neutralisation, 
 achieved by addition of water to the urethane acrylate or by addition of the urethane acrylate to water, a chain extension by means of component F) taking place in a third reaction step before, during or after the dispersion. 
 
     
     
       2. The process of  claim 1 , wherein one or more reactive diluents comprising at least one radically polymerizable group, component (ii), are added. 
     
     
       3. The process of  claim 1 , wherein component E) is dimethylolpropionic acid. 
     
     
       4. The process of  claim 1 , wherein, after the first reaction step, after determination of the NCO content, an NCO value is reached which is within 1.0 wt. % NCO (absolute) of the theoretical NCO content, and after the second reaction step, after determination of the NCO content, an NCO value is reached which is within 1.0 wt. % NCO (absolute) of the theoretical NCO content. 
     
     
       5. The process of  claim 1 , wherein component A) is selected from the group consisting of a polyester (meth)acrylate and a polyether ester (meth)acrylate. 
     
     
       6. The process of  claim 5 , wherein the molar ratio of isocyanate groups in D) to groups reactive towards isocyanates in A) to C) is from 1.5:1.0 to 3.0 to 1:0. 
     
     
       7. The process of  claim 1 , wherein component C) is a monomeric monool, diol and/or triol in each case having a molecular weight of from 32 to 240 g/mol. 
     
     
       8. The process of  claim 1 , wherein component A) is selected from the group consisting of a polyester (meth)acrylate and a polyether ester (meth)acrylate; and component C) is a monomeric monool, diol and/or triol in each case having a molecular weight of from 32 to 240 g/mol. 
     
     
       9. The process of  claim 8 , wherein the molar ratio of isocyanate groups in D) to groups reactive towards isocyanates in A) to C) is from 1.5:1.0 to 3.0 to 1:0. 
     
     
       10. The process of  claim 1 , wherein the molar ratio of isocyanate groups in D) to groups reactive towards isocyanates in A) to C) is from 1.5:1.0 to 3.0 to 1:0. 
     
     
       11. A radiation-curable, aqueous dispersion comprising a polyurethane acrylate (i) prepared by the process of  claim 1 . 
     
     
       12. A coating, lacquer, or adhesive comprising the radiation-curable, aqueous dispersion comprising a polyurethane acrylate (i) of  claim 11 . 
     
     
       13. A process for preparing a radiation-curable, aqueous dispersion comprising a polyurethane acrylate (i) comprising as structural components
 A) is selected from the group consisting of polyester (meth)acrylate and polyether ester (meth)acrylate, wherein the one or more polymeric compound has an OH number in the range from 15 to 300 mg KOH/g of substance; 
 B) optionally one or more monomeric compounds other than A) comprising a hydroxy function and at least one (meth)acrylate group; 
 C) is a monomeric monool, diol and/or triol, other than A) or B), in each case having a molecular weight of from 32 to 240 g/mol; 
 D) one or more organic polyisocyanates selected from the group consisting of 1,6-hexamethylene diisocyanate, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate or IPDI) and 4,4′-diisocyanatodicyclohexylmethane; 
 E) one or more compounds selected from the group consisting of dimethylolpropionic acid and dimethylolbutyric acid; 
 F) one or more compounds selected from the group consisting of mono-amines, di-amines, mono-functional amino alcohols and difunctional amino alcohols 
 wherein components A) through D) are reacted in a first reaction step to form a polyurethane acrylate which comprises neither ionic nor potentially ionic groups and, after determination of the NCO content, an NCO value is reached which is within 1.5 wt. % NCO (absolute) of the theoretical NCO content, wherein the molar ratio of isocyanate groups in D) to groups reactive towards isocyanates in A) to C) is from 1.2:1.0 to 4.0:1.0; and 
 in a second reaction step, component E) is reacted with free NCO groups from the reaction product of components A) to D) and, after again determining the NCO content, an NCO value is reached which is within 1.5 wt. % NCO (absolute) of the theoretical NCO content, wherein the molar ratio of isocyanate groups in D) to groups reactive towards isocyanates in A), B), C) and E) is from 1.2:1.0 to 1.5:1.0, wherein a neutralising agent is added during or after the second reaction step; and 
 in a third reaction step, component F) is reacted with free NCO groups, 
 wherein a dispersing step is carried out after the second step or, if the neutralisation is carried out after the second step, after neutralisation, 
 achieved by addition of water to the urethane acrylate or by addition of the urethane acrylate to water, a chain extension by means of component F) taking place in a third reaction step before, during or after the dispersion.